FNIRSI HRM-10 Battery Internal Resistance Tester

The capacity of a battery tells you how much is stored in it. The internal resistance tells you how fast it can give that back — and whether the cell is alive at all. An 18650 can read an honest 4.1 V and still sag to cut-off under load, because its internal resistance is 300 mΩ where it used to be 30.

The HRM-10 measures both at once: clip it on, and you see the voltage and the resistance. Measuring milliohms with an ordinary multimeter is pointless — the resistance of its own leads is larger than the thing you are trying to measure.

Why four wires

The clips are not ordinary ones but Kelvin clips: each jaw has two separate contact pads. One pair of wires carries the current, the other measures the voltage right at the battery terminal. The resistance of the leads, of the clips and of the oxide on the terminal never enters the result. That is why this instrument sees tenths of a milliohm and a multimeter does not.

What it is good for

• Sorting used 18650 and 21700 cells: a dead one has several times the internal resistance of a live one, even when both show a normal voltage
• Matching cells for a pack: cells put in parallel or in series should have similar resistance, or one of them does the work for everybody and gets hot
• Checking spot welds on nickel strip in a pack — a bad weld shows up as extra milliohms
• Checking contacts, connectors, fuses and traces: it is also simply a milliohmmeter
• Diagnosing a lead-acid battery without taking it out of the car

How to use

1. Switch on and make sure what you are measuring is DC. Never apply AC voltage, and the DC limit is 100 V.
2. Clip on so that both pads of each jaw touch metal. If one pad hangs in the air, the four-wire method quietly becomes a two-wire one and the reading drifts off.
3. Let the reading settle and read both values.
4. When sorting cells, measure them under the same conditions: same state of charge, same temperature, same spot on the terminal. Only then is the comparison worth anything.
5. Save the results you need to memory and export them as a table afterwards.

What not to do

• Do not connect it to mains or to any AC voltage.
• Do not measure the resistance of a circuit with something running in it: the instrument injects its own current.
• Do not bridge both terminals of a battery with the clips while in voltage mode — that is not a measurement, that is a short circuit through a clip.

Renting it

This one goes home with you: the usual reason is going through a box of used 18650 cells and working out which of them are still worth anything. The daily rate and the term are in the rental block on this page.

Access

The instrument is in a container on the shelf in the hackspace room, so a plan that includes hackspace access is required. No induction is needed.